In an image-capturing apparatus, such as a digital still camera, a digital video camera, and the like, equipped with a solid-state imaging sensor such as a CCD image sensor, a CMOS image sensor, and the like, there is performed white balance adjustment for adjusting gains of RGB signals in accordance with the color temperature of a light source so that a white substance can be recorded as a substance of real white color through image-capturing.
Incidentally, a comparatively bright area and a comparatively dark area are mixedly present in an image captured by the image-capturing apparatus. When the gains of the RGB signals are adjusted in accordance with the dark area, there is a possibility of the color of the light source failing to be reflected on the adjustment. When information about the dark area is used for gain adjustment, the information may become a factor responsible for an error in color reproduction.
Japanese Patent Laid-Open Publication No. 2000-92509 describes a technique of preventing the information about a dark area, to which the color of the light source is considered not to be reflected much, from affecting the adjustment of a gain for white balance. More specifically, Japanese Patent Laid-Open Publication No. 2000-92509 describes a technique of: dividing an image into a plurality of blocks; taking, as a typical value of a block, an average value including luminance and a color difference of each of RGB components determined on a per-block basis and determining a block of the highest luminance among the blocks in accordance with the typical value; determining a luminance threshold value from the luminance of the block of the highest luminance; determining a gain from the typical value of a block, among all of the blocks, whose luminance is higher than the luminance threshold value; and adjusting white balance in accordance with the gain.
As mentioned above, gains determined by placing high priority on the information about a comparatively-bright area are imparted to the respective RGB signals pertaining to the entire image, and white balance adjustment is performed. When a characteristic (hereinafter called an “output characteristic”) of the level of an output signal corresponding to the quantity of light entering a solid-state imaging sensor does not depend on the quantity of incident light but exhibits a linear relationship, color reproduction is not adversely affected much even when white balance is adjusted in connection with the RGB signal pertaining to the entire image by use of the gain determined by placing priority on the information about the comparatively-bright area (i.e., an area where the quantity of incident light is comparatively large). However, for instance, in reality, the output characteristic of a dark area where the quantity of incident light is small becomes nonlinear. Consequently, when white balance of the entire image including a lot of dark areas; e.g., a night view, is adjusted by use of the gain determined by placing priority on the information about a bright area, there may arise a case where color reproduction of the dark areas is adversely affected.
Japanese Patent Laid-Open Publication No. 2003-23642 describes a technique of providing an image-capturing apparatus with linearity correction means which determines, in advance, reference data, representing an input/output characteristic of a CCD in the form of an approximate line, by means of measurement; which detects the level of an output from the CCD in accordance with the quantity of incident light; and which corrects the characteristic of the output from the CCD in accordance with the reference data, thereby improving a shift in white balance.